Any rotor system, its mounting or controls, or the drive connection including shafting and direction-changing gearboxes can suffer from impending or catastrophic failure. The failure cause can be internal, typically a component failure, or external by the sustaining of battle damage. In any event, the method of monitoring the loads, vibrations, temperatures and rotational speeds will determine whether the failure is progressive or instantaneous, and whether the drive disconnection should be elective or automatic.
When in forward flight, the likelihood of secondary damage to an aircraft structure due to severe vibration from, for example, the loss of a rotor blade must be minimized. Aircraft with multiple lifting-rotors always have a mechanical linkage between the rotors to prevent lift imbalance should one rotor fail to rotate. The problem with that arrangement, however, is that the safety benefit in vertical flight becomes a liability in forward, wing-borne flight.
Until very recently, the range/payload capability of a tilt-rotor or compound aircraft was so limited that attention was not focused on this problem. More recently, a combination of gains in aircraft structural material strength, aerodynamic and dynamic improvements in rotors and in the fuel efficiency of gas turbine engines has resulted in a substantial improvement in VTOL aircraft utility. As the utility improves, and as the scale and payload capability of VTOL aircraft enlarges, (i.e. a decrease in the operating cost per ton-mile), it is clear that tilt-rotor aircraft can become part of a short to mid-range transportation system for passengers and freight. Increased attention to the tilt-rotor aircraft type is based on its potential for reducing airport congestion and at the same time achieving turbo-prop speed and fuel efficiency over ranges of several hundreds of miles. The vertical take off and conversion phase of a typical flight could now occupy only about one percent of total flight time. Given the mechanical complexity of tilt-rotor aircraft, and the new reality that they are cruise flight rather than hovering machines, a fresh appraisal of the flight safety implications of the engines/rotors/drive systems and their fail-operational behavior is needed. This is the subject and scope of the described invention.
In addition, during the last six decades of incremental helicopter development, design attention was always paid to their flight safety as rotor-borne lifting machines, whether for emergency rescue, the lifting of loads, or for military excursions. A prime objective was to minimize the effect of power loss leading to the use of two or more engines, and to the permanent shaft connection between the two rotors of a tilt rotor, or the two rotors of a tandem rotor, so that there was no sudden lateral or longitudinal lift imbalance in the event of one engine failing.
Most tilt-rotor, tilt-wing aircraft and multi lifting rotors compound helicopter designs rely on cross shafts between their 2 or 4 rotors to provide hover lift from all rotors when an engine fails. These cross shafts, in current design practice, are permanently engaged in flight such that all rotors turn if any engine is operational. Automatic disengagement of non operational engines is provided by the use of one-way clutches (Sprag-type clutches in most cases).
Propeller-driven passenger aircraft, either the piston engine powered examples of the 1920-60 era or the modern turboprop powered aircraft, don't hover or VTOL and therefore use no cross shaft. They are capable of safe flight and landing in the case of an engine failure or the mechanical or structural failure of a propeller. In such cases the propeller is stopped and the blades controlled to a streamline position (“feathered” in the aircraft vernacular).
During hover and conversion to forward flight, all known tilt-rotor and tilt-wing aircraft cannot continue flight to a safe landing when a rotor (not an engine) fails. However, as previously noted, these two flight regimes occupy a small and decreasing percentage of the total flight time as the utility of the aircraft improves and it becomes predominantly a transportation machine and not a lifting device. In order to render tilt-rotor and tilt-wing aircraft acceptable for large scale transportation of passengers, it must be possible to continue wing-borne flight, in airplane mode, to a safe landing with a damaged or disabled rotor, and not to allow this single point failure to unduly compromise flight safety.
Thus, there is still a need for methods and apparatus in which a failure of one rotor in a multiple lifting-rotor aircraft not interfere with rotation of the other rotor(s).